DREAM—a novel approach for robust, ultrafast, multislice B₁ mapping

Abstract

A novel multislice B₁-mapping method dubbed dual refocusing echo acquisition mode is proposed, able to cover the whole transmit coil volume in only one second, which is more than an order of magnitude faster than established approaches. The dual refocusing echo acquisition mode technique employs a stimulated echo acquisition mode (STEAM) preparation sequence followed by a tailored single-shot gradient echo sequence, measuring simultaneously the stimulated echo and the free induction decay as gradient-recalled echoes, and determining the actual flip angle of the STEAM preparation radiofrequency pulses from the ratio of the two measured signals. Due to an elaborated timing scheme, the method is insensitive against susceptibility/chemical shift effects and can deliver a B₀ phase map and a transceive phase map for free. The approach has only a weak T₁ and T₂ dependence and moreover, causes only a low specific absorption rate (SAR) burden. The accuracy of the method with respect to systematic and statistical errors is investigated both, theoretically and in experiments on phantoms. In addition, the performance of the approach is demonstrated in vivo in B₁-mapping and radiofrequency shimming experiments on the abdomen, the legs, and the head on an eight-channel parallel transmit 3 T MRI system.

Optimal control design of turbo spin‐echo sequences with applications to parallel‐transmit systems

“…The relative $B_1^{+}$ maps for the eight channels are derived from low flip‐angle gradient‐echo images, as described in 30. In addition, an absolute $B_1^{+}$ mapping measurement is performed with the DREAM method 31 and is used to recover quantitative information about the flip angle; the resulting maps are plotted in Figure 5a. We design optimal amplitude and phase settings for a three‐dimensional‐turbo‐spin‐echo scan (3D‐TSE, also known as SPACE, or VISTA), TSE factor = 113.…”

“…The $B_1^{+}$ maps for the eight channels are measured in the same way as in the previous test 30, 31 and they are displayed in Figure 7a. Constraints on peak and total RF power are also required.…”

Clinical applications of dual‐channel transmit MRI: A review

“…However, this is an area of active research, with several new schemes being developed which result in higher sensitivity, more rapid data acquisition and lower SAR: these include using the Bloch-Siegert effect 55 and very rapid dual-stimulated echo approaches. 13 Many of the current sequences are very accurate in mapping areas in which the transmit field is high, but are relatively inaccurate in areas with very low transmit efficiency, which are of course the areas that most need to be improved. B 1 - mapping sequences with greater "dynamic range" are, therefore, needed to improve the B 1 calibration quality, and consequently the B 1 shimming performance.…”

“…A short additional calibration scan (typically lasting less than a minute) is added to the start of the clinical protocol to map the B 1 field within the patient, and based upon this scan the optimum relative phases and magnitudes feeding the two ports of the dual-channel transmit coil are calculated. There are many different schemes for fast B 1 mapping [12][13][14] with different vendors using different sequences.…”

Clinical Functional MRI of the kidneys

“…However, dielectric effects caused by B 1 field interferences may result in local signal voids and mimic pathological lesions [5]. With radiofrequency (RF) shimming and parallel RF transmission, techniques have been developed within the last years providing more homogeneous flip angle distributions within the abdomen [6].…”